This invention relates to systems for the width control of an adjustable optical lit in response to the product of variable functions, one of which is programmed, such as in a monochromator forming a part of a spectrophotometer or like instrument, an infrared spectrophotometer in particular, wherein provision is included for multiplying an instantaneous programmed slit width by a variable factor under the control of the operator. It is desirable to meet the following practical requirements by a slit width control system incorporated in an infrared spectrophotometer.
In a spectrophotometer, the monochromator serves to analyze the wavelengths of the radiation transmitted, or absorbed, by a sample under analysis. The radiation beam passing through the monochromator is geometrically defined by an entry slit and an exit slit, usually integrated in a slit assembly for practical convenience. Each slit is formed by a pair of jaws, one of which at least may be displaced to control the slit width in accordance with certain operational requirements. The two slits are usually ganged for operation in concert.
Increasing the width of both slits naturally allows more energy to pass through to the detector and, consequently, the signal-to-noise ratio of the detector output is improved, but at the cost of reduced resolution. The right compromise between resolution and signal-to-noise ratio depends to a great extent on the circumstances of the analysis being performed, and this means that the operator must be provided with means for adjusting the slit width either in predetermined selectable steps or continuously.
Monochromators may be arranged to scan in the direction of either increasing or decreasing wavelength. If the slits are kept at the same width during the entire scan, the energy reaching the detector must vary as the scan proceeds, because of the wavelength dependent characteristics of the optical elements. This impairs the photometric accuracy, because of the inevitable lowering of the signal-to-noise ratio, as less energetic regions of the spectrum are scanned. Programming of the slit width for constant energy at the detector is a known and accepted way of overcoming this problem.
The need may now be appreciated for means enabling the operator's slit width control to be superimposed on the automatic control. In effect, this means that the programmed slit width must be multiplied by a factor which may vary either continuously or in steps, depending on whether the spectrophotometer design is such that the operator may select any control setting of his choice in a given range or only certain prededetmined settings.
Now, a slit program must naturally account for the grating constant and the angle of blaze of the grating in use and, in addition, the optical efficiencies of the other elements forming part of the photometric system of the spectrophotometer. In multi-grating instruments, this is taken care of by having multi-track slit control cams, each track being designed in relation to one grating. Where one grating is used in more than one order, each order must likewise be associated with its own track.
A mechanical complication arises in multiplying the displacement of the slit-cam follower by the factor referred to earlier. In some heretofore known spectrophotometers, linkages have been used involving variable fulcrum levers. Such linkages are comparatively simple and inexpensive to produce, but do not allow accurate multiplication and are only tolerably satisfactory within a comparatively narrow range.
It is, of course, well known in spectrophotometry that the ordinate of the recorded spectrum may be given in either transmission or absorption values and the abscissa in either wavelength or wavenumber values. In the present specification, the scanning function of the monochromator will be referred to, while assuming a wavenumber presentation of the abscissa, which is the one most commonly used in infrared instruments. For simplicity, the wavenumber scan will be referred to, although the monochromator in fact scans the wavelength of the radiation emerging from the sample. It must be understood, however, that the wavelength presentation is not excluded from within the scope of the present invention.
Normally, the use of multi-track slit control cams, as in known spectrophotometer monochromators, involved a design compromise brought about by the step-like change in the profile between the outgoing track and the incoming track at the grating or order change. In prior art slit control systems, the change was allowed to take place by actuating the scan drive through a small angle, after temporarily immobilizing the pen servo, and the chart drive, of course. Now, if the angle was made too small, the slit-cam follower could only negotiate the step between the two tracks in the forward scan drive but not in the reverse drive, which was an unacceptable limitation in a commercial instrument. If the angle was made too large, so much of the cam profile was used up that the incoming track had to be given an undesirably high rate of profile change if the scan was to be resumed either from the wavenumber reached just before a change was commanded or from a value slightly higher to ensure a small overlap region (it is assumed in the latter case, the pen servo was to be reactivated to coincide with the repassing of said value upon resumption of the wavenumber scan). The time taken to effect the change has always been, of course, another important factor. Ideally, it should be very brief, but the considerable inertia of the scan drive system has meant that hitherto changeover times in terms of seconds rather than fractions of a second have had to be tolerated.